1. Field of the Invention
The present invention relates to a drive apparatus for a vehicle in which one pair of front or rear wheels are driven by a vehicle power train and the other pair of the front or rear wheels are driven by a pair of electric motors coupled to the other pair of the front or rear wheels.
2. Background Information
In a conventional four-wheel drive vehicle that only has an engine as a power train, a space for accommodating a propeller shaft extending from a transmission in the front of the vehicle to the rear wheels is required. Therefore, in the conventional four-wheel drive vehicle, a propeller shaft tunnel is provided at a substantially widthwise center portion of a floor of the vehicle under the rear seats. In such a case, the floor is not level, and thus, comfort for the passengers is reduced. Also, another type of conventional four-wheel drive vehicle includes an arrangement in which the driven wheels are disengaged with an electromagnetic clutch or the like when four-wheel drive is not required. In other words, in this type of the conventional four-wheel drive vehicle, the driven wheels are disengaged from the power train and driven by the drive wheels during two-wheel drive mode. This type of conventional four-wheel drive vehicle has had problems in that the propeller shaft, a differential gear and the like continue to be rotated due to the forced rotation of the driven wheels during two-wheel drive mode. Therefore, fuel efficiency decreases in proportion to the friction of the propeller shaft and the differential gear.
In recent years, hybrid four-wheel drive vehicles (hereinafter referred to as hybrid 4WD vehicles) have been developed to solve some of the above mentioned problems of the conventional four-wheel drive vehicles. For example, Satoshi Chujo, et al., “Institute of Electrical Engineers, Technical Meeting on Vehicle Technology, Proceedings: 42 V Automobile Power Source Systems and Related Technology” (Dec. 6, 2002), publication No. VT-02-06 “e-4WD System” discloses a hybrid 4WD vehicle in which the front wheels are driven by an engine and the rear wheels are driven by an electric motor such as a DC motor. In this reference, the engine is also configured and arranged to drive a power generator at low to medium vehicle speeds. The hybrid 4WD vehicle disclosed in the above mentioned reference performs four-wheel drive using the engine and the DC motor with the power generated by the power generator when the vehicle starts moving or when the front wheels spin. When the engine is operating under other conditions, front-wheel drive (i.e., two-wheel drive) is performed using the engine alone.
In the hybrid 4WD vehicle disclosed in the above mentioned reference, the drive force of the DC motor is transmitted to the differential gear of the rear wheels via a reduction gear and an electromagnetic clutch, and the drive force is then distributed to the left and right rear wheels by the differential gear. When the hybrid 4WD vehicle disclosed in the above mentioned reference is driven in a two-wheel drive mode in which the drive force is generated by the engine alone, the torque-transmitting connections between the DC motor and the drive axles of the rear wheels (i.e., the driven wheels) are disengaged by the electromagnetic clutch to prevent the DC motor from idling with the rotation of the driven wheels.
Also in the hybrid 4WD vehicle disclosed in the above mentioned reference, most of the rear drive components such as the electrical wiring, the DC motor, the reduction gear, the electromagnetic clutch, the differential gear, and other components are disposed under the rear trunk. Therefore, the only difference between the hybrid 4WD vehicle disclosed in the above reference and a conventional front drive 2WD vehicle is that the height of the trunk bottom is slightly higher in the hybrid 4WD vehicle. Thus, with the hybrid 4WD vehicle disclosed in the above mentioned reference, there is almost no adverse affect on passengers' comfort in the rear seats. Accordingly, the hybrid 4WD vehicle disclosed in the above mentioned reference achieves four-wheel drive while the fuel consumption is kept lower than the conventional four-wheel drive vehicle in which the engine is the only drive source. Therefore, in the hybrid 4WD vehicle disclosed in the above mentioned reference, the vehicle weight is reduced due to the absence of a propeller shaft. Moreover, there is no idling friction in the propeller shaft, differential gear, and the like when the vehicle is in the two-wheel drive mode.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for a drive apparatus for vehicle. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.